The loosening of threaded spin couplings as a result of shock and vibration has long been recognized as a common cause of failure in adapters for electrical cabling and conduits. The historical methods used to solve this problem fall into two general categories, retention methods and locking methods.
Retention methods utilize some means of increasing the torque required to remove the spin coupling. Examples include adhesive coatings applied to threads, and mechanical detent devices which use a racheting mechanism to resist rotation in the coupling. These retention methods have the advantage of being easy to install, however, none provide a positive lock which will prevent coupling rotation. Both of these type of devices are subject to loosening in extreme operating conditions.
Locking methods utilize some means of preventing the spin coupling from rotating around an adapter body. The most common example of a locking method is the use of lock wires wherein a piece of wire is secured to the coupling and a fixed object thus preventing rotation. Lock wire provides a positive lock that prevents the coupling from loosening until the lock wire is removed. While effective in locking the coupling, the wire is cumbersome to install and repair.
Inventors have created several types of coupling locking devices that are integral with the coupling. U.S. Pat. No. 5,192,219 to Fowler et al. (1993) discloses a locking device that utilizes spring fingers to prevent rotation of the coupling; however, these spring fingers are expensive to manufacture, and result in a longer than normal coupling. The locking couple of U.S. Pat. No. 5,366,383 to Dearman (1994) is also much longer than a normal coupling.
A locking coupling must have a latching device that ensures that the lock remains locked, and is impervious to shock and vibration. The force required to engage and disengage the latch is critical to its performance under shock and vibration. Both of the above integral locking devices use an axial motion to engage and disengage the lock which limits the latching force to that of the finger strength of the installer, without having to use special tools. This factor is a clear disadvantage over a rotational locking motion which allows the installation tool to be used as a latching tool as well.
Current locking mechanisms can be damaged by improper removal by persons unfamiliar with the design. Repair facility personnel unfamiliar with the locking device can permanently damage the locking system by attempting to remove the coupling without first unlocking the coupling.